The described application relates generally to methods and systems involving removal of selenium, particularly removal of selenium from water containing one or more forms of selenium.
Selenium is an essential element in trace amounts in humans and animals. Unfortunately, in high amounts, selenium is often toxic. Water irrigation of alkaline soils has lead to selenium removal, which then concentrates in downstream water systems. Selenium present in coal is released after burning and accumulates in waste ponds.
Selenium often occurs in water in form of selenocyanate (SeCN−, a zero valance state), selenate (SeO4−2, a +6 valance state) or selenite (HSeO3 or SeO3−2, a +4 valance state). Under strongly reducing conditions, it appears as selenide (HSe−). Selenate is more common in water that is alkaline and oxidizing. Selenite is more common in water that is acidic and only moderately oxidizing. Selenium, including selenate, has proven to be difficult to remove from solution. Selenate, for example, is very soluble with no known precipitants and cannot be removed by adsorption or filtration. Most conventional methods for removing forms of selenium include desalting, absorption, chemical reduction, or using biologics. Most removal methods are interfered with by other competing anions in solution (e.g., silicate, bicarbonate, sulfate). Thus, selenium removal methods are generally mildly or moderately operative with only one form of selenium. Selenium removal systems also generally produce high waste volumes (e.g., sludge), are slow, leave very large carbon footprints and/or are very expensive.